The use of mineral fillers for thermoplastic polymer compositions is well known in the art. Such filler compositions are typically used to improve certain physical properties of the matrix polymer. Since mineral fillers often are as costly on a volume basis as the low cost resins such as the polyolefins, fillers are mostly used for the purpose of creating new materials of increased value, due to the altered physical properties as compared to the unfilled polymer matrix.
The filled thermoplastic polymers typically are comprised of the matrix polymer, a mineral filler, and an interface agent. Of importance then is the combination of properties that can be achieved for a given polymer/filler/interface agent system, such combination of properties typically being referred to as a property profile. The property profile thus determines the practical utility of the filled plastic.
The mineral fillers when incorporated into ductile plastics have some beneficial effects with respect to certain physical properties of the filled material while also causing some adverse effects on other aspects of the property profile of the composite material. In general, the addition of mineral fillers increases stiffness, as measured by tensile modulus and flexural modulus, while decreasing elongation at break, otherwise known as ductility, and impact strength, as measured by the notched Izod test. Such decreases in the ductility and impact strength are often severe. The strength properties which result from the addition of a mineral filler to a thermoplastic material may increase or decrease depending upon a number of factors.
Certain interface agents can dramatically improve the property profile of individual polymer/filler systems by lessening the deleterious effects of the filler while preserving the improvements in other properties, such as tensile modulus and flexural modulus. It is also sometimes possible to minimize by the use of certain interface agents, the adverse effects of the environment on a given composition, such as to improve its retention of properties after exposure to water, high temperature, and/or actinic radiation.
For a general background on the physical properties of polymeric materials, with and without particulate fillers, reference is made to the book by Lawrence E. Nielsen entitled "Mechanical Properties of Polymers and Composites", New York (Marcel Dekker) 1974, particularly to chapters 5 and 7, and to the appended bibliography. The specific question of how to simultaneously achieve high modulus, high ductility and impact strength in thermoplastics is discussed in more detail by L. C. Cessna in the article entitled "Dilatometric Studies of Polymers Undergoing High and Low Rate Tensile Deformation", Polymer Engineering and Science, Vol. 14, #10, p. 696-701 (1974), and the reference quoted therein.
The foregoing studies suggest that microcavitation, i.e., microvoid formation during deformation, can result in an enormous increase in the internal surface area during deformation, and that this may be the predominant energy-absorbing mechanism even for unfilled, high modulus, ductile polymer systems at the high strain rates typical of impact testing. For studies of microcavitation in filled systems, further reference is made to the articles by Nam P. Suh et al. entitled "Strain-Rate Sensitive Tough Fibre-Reinforced Composites", J. of Materials Science, Vol. 12, pp. 239-250 (1977), and "Void Nucleation of Particulate Filled Polymeric Materials," pp. 46-48 of the preprints from the 34th Annual Technical Conference of the Society of Plastics Engineers, Boston, May 4-7, 1981, and the reference cited therein. These papers show that interposing a liquid (e.g., silicone grease) interface between a matrix resin and fibers or particular fillers, respectively, can indeed increase the toughness of these composites by facilitating the formation of microvoids. However, the authors clearly recognized the limitations of the approach by stating on page 250 of the former article that the viscous coating reduces the static properties of the composite such as its tensile and flexural strengths and suggested that the concept could best be utilized where the amount of one time energy absorption is the primary design parameter. The approach was useful for products which must be able to resist one impact event only without catastrophic failure, although the material has been damaged to the extent that it will not withstand a second event. Examples of such specialty applications were safety related products such as highway guard rails, safety helmets and circuit breaker boxes.
For a more general discussion of the different types of interface agents and their use in the prior art, reference is made to U.S. Pat. Nos. 4,385,136, issued May 24, 1983, entitled "Reinforcement Promoters for Filled Thermoplastic Polymers" and 4,409,342, issued Oct. 11, 1983, entitled "Synergistic Reinforcement Promoter Systems for Filled Polyolefins" and to the references quoted therein. Effective interface agents for mineral-filled thermoplastics fall into two broad categories, namely those that are chemically active and those that have no chemical groups capable of reaction with the matrix polymer. The first type is believed to work by a mechanism whereby the polymer interphase region surrounding each filler particle is chemically modified by the interface agent so that it becomes considerably tougher than the original matrix resin. The second type, on the other hand, is believed to debond the filler particles from the matrix resin, thus sometimes increasing the toughness by facilitating microcavitation, but usually at a severe sacrifice in both strength and stiffness. Actually, most additives that are presently used as dispersing and processing aids, i.e. most lubricants and surfactants, have only very minor effects on the mechanical properties of the filled polymer.
Many evaluations of additives for filled polymers described in the prior art are misleading because the data are incomplete or misinterpreted. For example, some additives act as plasticizers and thus reduce the stiffness of a given filled thermoplastic by softening the matrix which often is an undesirable effect. On the other hand, an increase in stiffness can be an indication of filler agglomeration--i.e. poor dispersion--hence, a high modulus is not necessarily a beneficial result for a particular additive agent as explained further below. In contrast, since agglomeration invariably reduces the tensile strength because poorly dispersed fillers cause large flaws, a high tensile strength is always a beneficial interface effect. The property profile which is important for practical utility is a combination of improved impact and ductility, unchanged or only slightly reduced strength and stiffness, and good retention of these properties after environmental exposure to moisture, heat or sunlight. This property combination is also the most difficult one to achieve, especially with interface agents that are incapable of chemical interactions with the matrix resin.
The relationship between particle aggregation and viscosity of suspensions was studied by T. B. Lewis and Lawrence Nielsen and reported in the Transactions of the Society of Rheology, Vol. 12, pp. 421-443 (1968). According to these studies, the viscosity of liquid dispersions (and similarly, the modulus of filled, solid polymers) decreases with improvements in the dispersion of particular fillers. The reason for this is that part of the liquid (or plastic matrix) is entrapped in the interstitial spaces in an agglomerate, hence reducing the amount of free liquid (plastic solid) available for the particles to move in. As the agglomerates are broken down, mechanically and/or by the action of dispersing agents, more liquid (solid) becomes available for the particles to move in and the viscosity (modulus) decreases.
The strength of a material is a statistical property in that the size and distribution of defects or flaws in the specimen determine the magnitude of loading a given sample can withstand before failure. For this reason, large filler particles quite generally result in lower composite strength than smaller ones, and in case of undispersed and mechanically weak agglomerates, the phenomenon is aggravated by the possible breakage of the agglomerates themselves.
Because of the complex mechanisms described above, the practical utility of a potential interface agent requires a quite complete evaluation of the physical property profile of the filler/polymer system in which it is used. Most studies reported in the literature are very imcomplete in this respect and considerable caution must therefore be exercised in accepting the statements of the authors.
For a perspective on the prior art for non-reactive interface agents, reference is made to some recent survey articles. Organic titanates have been reviewed extensively by S. J. Monte, G. Sugarman et al. in two papers entitled "The Theory of Organo Titanate Coupling Agents" and "The Application of Titanate Coupling Agents", pp. 27-34 and p. 35-39, respectively, in the preprints of the 34th Annual Technical Conference of the Society of Plastics Engineers, Atlantic City, N.J., Apr. 26-29, 1976. Chlorinated paraffins have been reviewed in a paper entitled "Low cost Filler-Coupling Agent for Polyolefins" by D. Stevenson et al., pp. 1-4 of the preprints for session 9-B of the 36th Annual Conference of the Reinforced Plastics/Composites Institute of the Society of the Plastics Industry, Washington, D.C., Feb. 16-20, 1981. An organic interface agent of undisclosed composition has been discussed by de Souza et al. in a paper entitled "Low Cost Highly Filled Impact Resistant Thermoplastic Composites," pp. 492-496 in the preprints of the 37th Annual Technical Conference of the Society of Plastics Engineers, New Orleans, La., May 7-10, 1979. A detailed overview of surface treatments and preparation techniques for calcium carbonate is presented in an article by T. Nakatsuka entitled "Surface Modification of Calcium Carbonate for Polymer Composites", published in H. Ishida and G. Kumar (ed.), "Molecular Characterization of Polymer Composites", New York (Plenum Press), 1985.
Non-reactive interface agents have some important advantages over the chemically reactive agents referred to above. First, precisely because the chemically non-reactive agents act by physical means only, they are generally much less sensitive to differences in processing conditions, thus making the same composite formulation usable for many different fabrication processes. Second, they are for the same reason generally very low in toxicity, which has advantages both in handling and in certain, critical applications of filled polymers. Third, in contrast to the chemically reactive agents, which usually do not give additive effects by simple combination with a non-reactive agent, all non-reactive species can often be combined with themselves with beneficial results: for example, highly hydrophobic interface agents will usually convey improved moisture resistance to a filled polymer when substituted for part of a less hydrophobic impact promoter without significant sacrifice in the other physical properties.
As polyethylene is one of the most widely used low cost resins, a need continually exists for improving the property profile of polyethylene based resins. It has been well known to use mineral fillers in combination with a polyethylene-type matrix polymer to increase its tensile modulus and flexural modulus. However, to date, there continues to exist a need for non-reactive interface agents, generally, and in particular for non-reactive interface agents which are capable of increasing the impact strength of such mineral-filled polyethylene-type resins while not substantially adversely affecting the strength and modulus of the filled polyethylene-type resin. The need continues to exist, in part, because of the lack of predictability of the efficacy of any individual non-reactive interface agent when combined with any particular combination of resin and filler.
One may refer, for example, to U.S. Pat. No. 4,385,136, which in Table III, Columns 11 and 12, lists a dozen typical, non-reactive chemicals. The subsequent examples demonstrate that such chemicals when used as additives in filled polyolefins typically do not provide composite materials with attractive and useful property profiles. For example, Example 1, Table 1 Column 19 shows that isopropyl tri-isostearyl titanate severely reduces strength, stiffness and impact, while increasing ductility in aluminum trihydrate filled polyethylene. As another illustration, Example 3, Table 3, Column 20, demonstrates that iso-stearic acid, while improving the impact strength, has little effect on the strength, stiffness, and ductility over a control sample without such an additive, in clay filled high density polyethylene.
An object of the present invention is to provide non-reactive impact promoters which are useful in conjunction with certain combinations of mineral fillers and resins.
Another object of the present invention is to provide non-reactive impact promoters which are useful in increasing the impact strength of certain mineral filled resins, when compared to the same compositions without said promoter.
Additionally, an object of the present invention is to provide a method for improving the impact strength of a mineral filled resin composition without substantially impairing the strength and modulus of said composition.
A further object of the present invention is to provide a method for concurrently reducing the moisture sensitivity of a mineral-filled resin composition while improving the impact strength of said composition, without substantially impairing the strength and modulus of the composition